I think there is an additive to wine that goes in just before bottling to prevent further fermentation.
What is it? How much should I use for a 5 gal carboy?
Do the commercial ppl use this all the time?

You might be referring to potassium sorbate. It's used mainly when there is residual sugar in the wine. You have to be careful with it--it makes wine taste like Lifesavers. Additions of it depend on how much sugar is in the wine plus you must adjust sulfur dioxide levels in conjunction with sorbate additions.

The reason I asked this is because the last batch I made seemed done fermenting.
I bottled after 3 1/2 months and at first, there was some "fizzing".
There were a few loose corks.
Someone told me that there was an additive to stop extra fermentation.
How do I check the alcohol level in wine?

It's easier to check for sugar than for the alcohol level--that takes an expensive piece of equipment. Is the wine red or white? If white, you can check sugar with the tablets that diabetics use to check sugar in urine.
Ask your wine supplies guy what he has to check for sugar. He'll also have the sorbate if you need it.